(a) Field of the Invention
The present invention relates to a method of manufacturing electret filters for efficiently collecting air-borne dusts by means of an electrostatic mechanism.
(b) The Prior Art
Methods for manufacturing electret filters taught by the prior art include:
(1) A method involving covering either a mass of layered fibers or an electrode with an electrically permeable film and alternately heating and cooling while applying a high voltage electric field to provide a semi-permanent electric charge (U.S. Pat. No. 3,571,679, Japanese Patent Gazette (Kokoku) No. 4433/1974).
(2) A method for forming a mass of layered fibers by heating a polymeric film to a temperature close to its melting point and passing the heated film through a high voltage electric field provided between positive and negative charge-generating electrodes while stretching the film to provide a semi-permanent electric charge. The charged film is then fibrillated into fine fibers by passing over a needle roller (U.S. Pat. No. 3,998,916).
(3) A method for forming a mass of layered fibers by spraying charged particles onto polypropylene fibers while above their melting point, as they are extruded from orifices by melt-blow spinning into a high speed hot air stream, prior to solidification of the fibers (Japanese Patent Application Laid-Open No. 113900/1979).
(4) A method for imparting an electric charge to a filter material in which a high voltage is impressed across a layered composite of electro-conductive fibrous sheets of cellulose fiber etc. placed on each side of the filter material which, in turn, is composed of fibers of a thermoplastic such as polypropylene. The high voltage is generated by means of at least one pair of corona discharge bars placed on the opposite sides of the assembled sheets, in close proximity thereto (U.S. Pat. No. 4,375,718).
Conventional manufacturing methods (1), (2) and (3), however, have the disadvantage that the fibers must be charged after being heated to a temperature close to their melting point. The advantages of charging at room temperature or cold charging over hot charging are set forth at col 2, lines 6-64 of U.S. Pat. No. 4,375,718. In addition, the method (1) which requires that the electrodes be covered with a weakly conductive asbestos cement is problematic because of the long period of time required for charging to the desired degree. Method (2) in which a mass of layered fibers is formed from a previously electretized film material is excellent in providing the fibers a semi-permanent charge but problematic in that the production process is complicated, requires a long period of time, and is costly. Further, in (2) the thickness of fiber to be produced is limited. Method (3) in which fibers are electrostatically charged as they are extruded through orifices, encounters a problem in that the electrostatic charge imparted to the fibers while still molten is neutralized in part before the fibers solidify and thereby prevented from reaching a sufficient level.
Method (4) offers the advantage that the fibrous sheet can be electrostatically charged in a short time at approximately room temperature. However, it is accompanied by certain disadvantages including the dependency of the amount of electric charges induced or conveyed to the filter material on the condition of contact between the filter material and the electroconductive sheet, and the poor reliability of filtration efficiency due to the tendency of the electric charges to stay only on the fiber surface of the filter material rather than permeate into the interior of the fiber. Further, this method suffers from the disadvantage that migration of the charged particles (ionized air) generated by the corona discharge depends on the electrostatic condition of the filter material and the electroconductive webs, all of which are constantly in frictional contact with the rollers etc.